1. Field of the Invention
The present invention relates to a variable-shape mirror that permits the shape of a mirror surface thereof to be varied, and more particularly relates to the structure thereof. The present invention also relates to an optical pickup device provided with such a variable-shape mirror.
2. Description of Related Art
When information is read from or written to an optical disc such as a CD (compact disc) or DVD (digital versatile disc) by the use of an optical pickup device, the relationship between the optical axis of the optical pickup device and the disc surface should ideally be perpendicular. In reality, however, when the disc is rotating, their relationship does not always remain perpendicular. As a result, with an optical disc such as a CD or DVD, when its disc surface becomes inclined relative to the optical axis, the optical path of laser light is so bent as to produce coma aberration.
When coma aberration is produced, the spot of laser light shone on the optical disc deviates from the proper position, and, when the coma aberration becomes larger than permitted, inconveniently, it becomes impossible to accurately write or read information. For this reason, there have conventionally been proposed methods for correcting coma aberration as described above and other aberrations by the use of a variable-shape mirror.
For example, JP-A-2004-070004 proposes a method of reducing wavefront aberrations in a variable-shape mirror employing piezoelectric elements and having a unimorph or bimorph structure. The variable-shape mirror here has, for example, a structure as shown in FIGS. 7A and 7B. FIG. 7A shows the variable-shape mirror with the mirror fitting member 108 thereof removed, as seen from the side opposite to the mirror material 101 thereof. FIG. 7B is a sectional view as cut along line A-A shown in FIG. 7A. The variable-shape mirror comprises piezoelectric elements 102, wiring electrodes 103, individual electrodes 104, a mirror base 105, fixed portions 106, and slits 107.
According to JP-A-2004-070004, when, with the wiring electrodes 103 grounded, a positive voltage is applied to one of the individual electrodes 104 and a negative voltage to the other, one of the piezoelectric element 102 expands and the other contracts. Thus, the mirror surface becomes convex in the part thereof located on one side of the center thereof in the A-A direction and concave in the part thereof located on the other side. It is described that using this variable-shape mirror in an optical pickup device helps reduce wavefront aberrations.
However, JP-A-2004-070004 reports that, in a variable-shape mirror having such a structure, deformation is produced in such locations as the slits 107 arranged on the surface of the mirror material 101 when the shape of the mirror surface is varied. To correct this, JP-A-2004-070004 proposes a variable-shape mirror provided with an elastic structure arranged in the vicinity of where the deformation is produced to thereby reduce the deformation of the mirror surface and enhance the variation efficiency of the mirror surface.
Among other conventional variable-shape mirrors, as shown in JP-A-H05-333274, there is a method of performing phase control by varying the shape of the mirror itself by the use of a plurality of actuators.
However, when, for example, a mirror surface of a variable-shape mirror is varied by sandwiching a piezoelectric element between a support base and a mirror portion and by exploiting a lateral displacement of the piezoelectric element placed on the support base, it has been found that deformation is produced, as shown by arrows in FIG. 8, on the mirror surface of the mirror portion on a side opposite to a side where the piezoelectric element and the mirror portion made contact with each other when the piezoelectric element is driven. FIG. 8 shows results of a computer simulation obtained through a finite-element method that is a common technique used as an approximation analysis technique for analyzing deformation or stress of a structure.
In other words, for example, as long as the shape of the mirror is varied by the contraction of the piezoelectric element as is the case with JP-A-2004-070004 or JP-A-H05-333274, local deformation is inevitably produced on the mirror surface at a location where the piezoelectric element is arranged. For this reason, if a variable-shape mirror structured as described in JP-A-2004-070004 or JP-A-H05-333274 is provided in an optical pickup device, local deformation is produced on the mirror surface. Consequently, it may not be possible to properly correct aberrations.